Improving Electron Mobility of Tetraphenylethene-Based AIEgens to Fabricate Nondoped Organic Light-Emitting Diodes with Remarkably High Luminance and Efficiency

Highly Efficient Multicolor-Emitting Tetraphenylethylene-Based Organic Salts with Commercialization Prospects

Since the discovery of aggregation-induced emission from tetraphenylethylene derivatives, various methods have been explored to prepare highly efficient multicolored luminescent materials. Herein, we report a simple and efficient strategy for constructing luminescent organic salts of the tetracationic luminogen, tetrapyridinium-tetraphenylethylene (T4Py-TPE4+), combined with seven di- and tetra-anionic aromatic sulfonate ligands. When aqueous solutions of the cationic luminogen and the anionic ligands were mixed, they rapidly aggregated into organic salts within seconds to minutes, giving yields of up to >90%. This was accompanied by an increase in the emission efficiency from ∼58% to almost 100%, and the ability to tune the emission color between 511 and 586 nm. These improvements were mainly attributed to the strong electrostatic attractions between the cation and anions, which resulted in the formation of a rigid hydrophobic network of the T4Py-TPE4+ luminogen with various π-conjugation lengths. Because these compounds are commercially available, this method opens the possibility of fabricating novel light-emitting materials for device fabrication and research.

Indolocarbazoles with Sterically Unrestricted Electron-Accepting Anchors Showcasing Aggregation-Induced Thermally Activated Delayed Mechanoluminescence for Host-Free Organic Light-Emitting Diodes

We investigated the effects of sterically nonrestricted electron-accepting substituents of three isomeric indolocarbazole derivatives on their aggregation-induced emission enhancement, mechanochromic luminescence and thermally activated delayed fluorescence. The compounds are potentially efficient emitters for host-free organic light-emitting diodes. The films of indolocarbazole derivatives exhibit emissions with wavelengths of fluorescence intensity maxima from 483 to 500 nm and photoluminescence quantum yields from 31 to 58%. The ionization potentials of the solid samples, measured by photoelectron emission spectrometry, are in the narrow range of 5.78-5.99 eV. The electron affinities of the solid samples are in the range of 2.99-3.19 eV. The layers of the derivatives show diverse charge-transporting properties with maximum hole mobility reaching 10-4 cm2/Vs at high electric fields. An organic light-emitting diode with a light-emitting layer of neat compound shows a turn-on voltage of 4.1 V, a maximum brightness of 24,800 cd/m2, a maximum current efficiency of 12.5 cd/A and an external quantum efficiency of ca. 4.8%. When the compounds are used as hosts, green electroluminescent devices with an external quantum efficiency of ca. 11% are obtained. The linking topology of the isomeric derivatives of indolo[2,3-a]carbazole and indolo[3,2-b]carbazole and the electron-accepting anchors influences their properties differently, such as aggregation-induced emission enhancement, mechanochromic luminescence, thermally activated delayed fluorescence, charge-transporting, and electroluminescent properties. The derivative indolo[3,2-b]carbazole displays good light-emitting properties, while the derivatives of indolo[2,3-a]carbazole show good hosting properties, which make them useful for application in electroluminescent devices.

Tetraphenylethylene sulfonate derivative as turn-on fluorescent sensor for berberine chloride detection in aqueous solution

A negatively-charged tetraphenylethylene derivative (TPE-SE) was designed and synthesized as turn-on fluorescent sensor for berberine chloride (BBC) detection in aqueous solution. The fluorescent property and detection mechanism were elucidated by UV-vis absorption spectra, photoluminescence spectra, dynamic light scattering experiments. The results reveal that the BBC can lead to aggregation-induced emission of TPE-SE due to the electrostatic interactions, endowing TPE-SE with excellent turn-on detecting ability, high selectivity and sensitivity to BBC. The detection limit is as low as 6.58 × 10^-6M. These results should be applicable to fabricate special turn-on fluorescent sensors towards various antibiotics, and it is crucially important for achieving reasonable control and intake of small biomolecules.

Hybrid Device of Blue GaN Light-Emitting Diodes and Organic Light-Emitting Diodes with Color Tunability for Smart Lighting Sources

A lighting device with a wide color-tunable range is still a challenge for lighting based on either organic light-emitting diodes (OLEDs) or inorganic LEDs. In this work, we first proposed a novel hybrid device of organic LEDs and inorganic blue GaN LEDs to achieve full white and other colors. Organic LEDs were stacked with green and red emissive layers and connected with blue GaN LEDs in parallel but in opposite polarity voltage. Under the alternate-current (AC) driving, the hybrid structure can be controlled independently by applying timing variable opposite voltages to emit the light from either blue LEDs or the stacked OLEDs for forming mixed colors. The hybrid device can generate white light, varying in a wide range by changing the amplitude and duty ratio (DR) of AC-driving signals, from cold white to standard white and to warm white (3668-11 833 K). When an AC voltage of (4.80 V, -2.45 V) was applied, the device has a high color gamut of 95.24% National Television System Committee (NTSC) and a high color rendering index (R _a) of 92.4%. The novel hybrid device with the blue LED and OLED in opposite polarity exhibits potential applications in smart solid-state lighting, display, and light communication.

Organic Single Crystals with High Photoluminescence Quantum Yields Close to 100% and High Mobility for Optoelectronic Devices

Organic single crystals with excellent optical and electrical properties are critical for the development of organic optoelectronics. Herein, two compounds 9,10-bis([N,N-diphenyl]-4'-phenylethynyl)anthracene (TPA-An) and 9,10-bis([1',3'-diphenyl]-5'-phenylethynyl)anthracene (TBA-An) are synthesized by introducing two different luminescent groups, triphenylamine and 1,3-diphenylbenzene, at the 9,10 positions of anthracene via triple bond connection. Single crystals based on TPA-An and TBA-An with a ribbon morphology obtained through the slow solvent-evaporation method exhibit high photoluminescence quantum yields (PLQYs) of 98% and 99% at room temperature, and remarkable hole mobilities of 0.45 and 0.15 cm² V^-1 s^-1 in single-crystal organic field-effect transistors (SC-OFETs). Furthermore, UV phototransistors based on the two single crystals obtain photosensitivities of 1.03 × 10³ and 3.45 × 10⁴ , ultrahigh photoresponsivities of 7.19 × 10⁵ and 1.50 × 10⁵ A W^-1 , and the detectivities exceeding 1.40 × 10¹⁶ and 1.60 × 10¹⁷ Jones.

Star-Shaped ESIPT-Active Mechanoresponsive Luminescent AIEgen and Its On-Off-On Emissive Response to Cu2+/S2

Design and development of multifunctional materials have drawn incredible attraction in recent years. Herein, we report the design and construction of versatile star-shaped intramolecular charge transfer (ICT)-coupled excited-state intramolecular proton transfer (ESIPT)-active mechanoresponsive and aggregation-induced emissive (AIE) luminogen triaminoguanidine-diethylaminophenol (LH3 ) conjugate from simple precursors triaminoguanidine hydrochloride and 4-(N,N-diethylamino)salicylaldehyde. Solvent-dependent dual emission in nonpolar to polar protic solvents implies the presence of ICT-coupled ESIPT features in the excited state. Aggregation-enhanced emissive feature of LH3 was established in the CH3CN/water mixture. Furthermore, this compound exhibits mechanochromic fluorescence behavior upon external grinding. Fluorescence microscopy images of pristine, crystal, and crushed crystals confirm the naked-eye mechanoresponsive characteristics of LH3 . In addition, LH3 selectively sensed a Cu2+ ion through a colorimetric and fluorescence "turn-off" route, and subsequently, the LH3 -Cu2+ ensemble could act as a selective and sensitive sensor for S2- in a "turn-on" fluorescence manner via a metal displacement approach. Reversible "turn-off-turn-on" features of LH3 with Cu2+/S2- ions were efficiently demonstrated to construct the IMPLICATION logic gate function. The Cu2+/S2--responsive sensing behavior of LH3 was established in the paper strip experiment also, which can easily be characterized by the naked eye under daylight as well as a UV lamp (λ = 365 nm).

A thermally activated delayed fluorescence exciplex to achieve highly efficient and stable blue and green phosphorescent organic light-emitting diodes

The development of a thermally activated delayed fluorescence (TADF) exciplex with high energy is of great significance in achieving highly efficient blue, green, and red organic light-emitting diodes (OLEDs) for use in full-color displays and white lighting. Highly efficient and stable blue and green phosphorescent OLEDs were demonstrated by employing a TADF exciplex (energy: 2.9 eV) based on 4-substituted aza-9,9'-spirobifluorenes (aza-SBFs). Blue PhOLEDs demonstrated a maximum current efficiency (CE) of 47.9 cd A-1 and an external quantum efficiency (EQE) of 22.5% at 1300 cd m-2 (2.5 times the values of aza-SBF-based systems), with the best blue PhOLED demonstrating a CE, power efficiency (PE) and EQE of 60.3 cd A-1, 52.7 lm W-1, and 26.2%, respectively. Green PhOLEDs exhibited a CE of 78.1 cd A-1 and EQE of 22.5% at 9360 cd m-2, with the best green PhOLED exhibiting a maximum CE, PE, and EQE of 87.4 cd A-1, 101.6 lm W-1, and 24.5%, respectively. The device operational lifetime was improved over 17-fold compared to reference devices because of the high thermal stability of the materials and full utilization of the TADF exciplex energy, indicating their potential for application in commercial OLEDs.

Theoretical study of charge-transport and optical properties of organic crystals: 4,5,9,10-pyrenedi-imides

This work presents a systematic study of the conducting and optical properties of a family of aromatic di-imides reported recently and discusses the influences of side-chain substitution on the reorganization energies, crystal packing, electronic couplings and charge injection barrier of 4,5,9,10-pyrenedi-imide (PyDI). Quantum-chemical calculations combined with the Marcus-Hush electron transfer theory revealed that the introduction of a side chain into 4,5,9,10-pyrenedi-imide increases intermolecular steric interactions and hinders close intermolecular π-π stacking, which results in weak electronic couplings and finally causes lower intrinsic hole and electron mobility in t-C5-PyDI (μh = 0.004 cm2 V-1 s-1 and μe = 0.00003 cm2 V-1 s-1) than in the C5-PyDI crystal (μh = 0.16 cm2 V-1 s-1 and μe = 0.08 cm2 V-1 s-1). Furthermore, electronic spectra of C5-PyDI were simulated and time-dependent density functional theory calculation results showed that the predicted fluorescence maximum of t-C5-PyDI, corresponding to an S 1→S 0 transition process, is located at 485 nm, which is close to the experimental value (480 nm).

Structure optimization of perovskite quantum dot light-emitting diodes

Although all-inorganic perovskite light emitting diodes (PeLED) have satisfactory stability under an ambient atmosphere, producing devices with high performance is challenging. A device architecture with a reduced energy barrier between adjacent layers and optimized energy level alignment in the PeLED is critical to achieve high electroluminescence efficiency. In this study, we report the optimization of a CsPbBr3-based PeLED device structure with Li-doped TiO2 nanoparticles as the electron transport layer (ETL). Optimal Li doping balances charge carrier injection between the hole transport layer (HTL) and ETL, leading to superior performance in both devices. The turn-on voltages for devices with Li-doped TiO2 nanoparticles were significantly reduced from 7.7 V to 4.9 V and from 3 V to 2 V in the direct and inverted PeLED structures, respectively. The low turn-on voltage for green emission is one of the lowest values among the reported CsPbBr3-based PeLEDs. Further investigations show that the device with an inverted structure is superior to the device with a direct structure because the energy barrier for carrier injection was minimized. The inverted structure devices exhibited a current efficiency of 5.6 cd A-1 for the pristine TiO2 ETL, while it was 15.2 cd A-1 for the Li-doped TiO2 ETL, a factor of ∼2.7 enhancement at 5000 cd m-2.

Facile Synthesis of a Terephthalic Acid-Based Organic Fluorophore with Strong and Color-Tunable Emission in Both Solution and Solid States for LED Applications

Dual-state emission (DSE) fluorophores with strong photoluminescence in both solution and solid states have wide applications in fluorescent probes and photoelectric devices. However, most of the existing DSE fluorophores involve complex synthetic strategies or only one or two emission colors in the solid state, which may hinder their practical applications. Herein, we report a facile and effective strategy to fabricate a kind of novel adjustable DSE fluorophore TPAA-Cu based on the small molecule terephthalic acid (TPA) and ascorbic acid (AA) with CuCl₂ as the catalyst. Not only can the TPAA-Cu solution emit blue, blue-green, and yellow colors depending upon the pH of solution, but also it shows reversible pH-tunable fluorescence colors. More importantly, corresponding TPAA-Cu solid fluorophores with blue, blue-green, and yellow colors can also be obtained with the quantum yields of 16.3, 26.7, and 29.6%, respectively.

All-Solution-Processed Pure Formamidinium-Based Perovskite Light-Emitting Diodes

All-solution-processed pure formamidinium-based perovskite light-emitting diodes (PeLEDs) with record performance are successfully realized. It is found that the FAPbBr₃ device is hole dominant. To achieve charge carrier balance, on the anode side, PEDOT:PSS 8000 is employed as the hole injection layer, replacing PEDOT:PSS 4083 to suppress the hole current. On the cathode side, the solution-processed ZnO nanoparticle (NP) is used as the electron injection layer in regular PeLEDs to improve the electron current. With the smallest ZnO NPs (2.9 nm) as electron injection layer (EIL), the solution-processed PeLED exhibits a highest forward viewing power efficiency of 22.3 lm W^-1 , a peak current efficiency of 21.3 cd A^-1 , and an external quantum efficiency of 4.66%. The maximum brightness reaches a record 1.09 × 10⁵ cd m^-2 . A record lifetime T₅₀ of 436 s is achieved at the initial brightness of 10 000 cd m^-2 . Not only do PEDOT:PSS 8000 HIL and ZnO NPs EIL modulate the injected charge carriers to reach charge balance, but also they prevent the exciton quenching at the interface between the charge injection layer and the light emission layer. The subbandgap turn-on voltage is attributed to Auger-assisted energy up-conversion process.

Interfacial Energy-Level Alignment for High-Performance All-Inorganic Perovskite CsPbBr3 Quantum Dot-Based Inverted Light-Emitting Diodes

All-inorganic perovskite light-emitting diode (PeLED) has a high stability in ambient atmosphere, but it is a big challenge to achieve high performance of the device. Basically, device design, control of energy-level alignment, and reducing the energy barrier between adjacent layers in the architecture of PeLED are important factors to achieve high efficiency. In this study, we report a CsPbBr₃-based PeLED with an inverted architecture using lithium-doped TiO₂ nanoparticles as the electron transport layer (ETL). The optimal lithium doping balances the charge carrier injection between the hole transport layer and ETL, leading to superior device performance. The device exhibits a current efficiency of 3 cd A^-1, a luminance efficiency of 2210 cd m^-2, and a low turn-on voltage of 2.3 V. The turn-on voltage is one of the lowest values among reported CsPbBr₃-based PeLEDs. A 7-fold increase in device efficiencies has been obtained for lithium-doped TiO₂ compared to that for undoped TiO₂-based devices.

A new blue AIEgen based on tetraphenylethene with multiple potential applications in fluorine ion sensors, mechanochromism, and organic light-emitting diodes

A new blue organoboron AIEgen with multiple potential applications is synthesized and studied.

Highly Efficient Nondoped OLEDs with Negligible Efficiency Roll-Off Fabricated from Aggregation-Induced Delayed Fluorescence Luminogens

Purely organic emitters that can efficiently utilize triplet excitons are highly desired to cut the cost of organic light-emitting diodes (OLEDs), but most of them require complicated doping techniques for their fabrication and suffer from severe efficiency roll-off. Herein, we developed novel luminogens with weak emission and negligible delayed fluorescence in solution but strong emission with prominent delayed components upon aggregate formation, giving rise to aggregation-induced delayed fluorescence (AIDF). The concentration-caused emission quenching and exciton annihilation are well-suppressed, which leads to high emission efficiencies and efficient exciton utilization in neat films. Their nondoped OLEDs provide excellent electroluminescence efficiencies of 59.1 cd A^-1 , 65.7 lm W^-1 , and 18.4 %, and a negligible current efficiency roll-off of 1.2 % at 1000 cd m^-2 . Exploring AIDF luminogens for the construction of nondoped OLEDs could be a promising strategy to advance device efficiency and stability.

Visible-Light Photoinduced Electron Transfer Promoted by Cucurbit[8]uril-Enhanced Charge Transfer Interaction: Toward Improved Activity of Photocatalysis

Visible-light photoinduced electron transfer (Vis-PET) is highly important for optoelectronic devices and photoredox catalysis. Herein, we propose a supramolecular strategy to promote the Vis-PET process by charge transfer (CT) interactions. As a proof of concept, a molecular system containing 1,5-alkoxy-substituted naphthalene and viologen moieties was designed to form a CT complex in water. The HOMO/LUMO energy gap was lowered by CT interaction, which turned on the Vis-PET process to generate viologen radical cations. Moreover, when CT interaction was enhanced by cucurbit[8]uril, the efficiency of the Vis-PET process was further promoted and the required irradiation wavelength could be further red-shifted by 100 nm. The Vis-PET system exhibited an improved activity of photocatalysis, as supported by the fast photoreduction of Cytochrome c. This study represents a facile supramolecular way to fabricate radicals with maintained activity under mild conditions, which holds potential to enrich the scope of visible-light photoredox catalysis by the rational utilization of CT systems.

Wettability with Aggregation-Induced Emission Luminogens

Aggregation-induced emission luminogens (AIEgens) have become an emerging field since the concept of AIE was proposed in 2001. Recently, AIEgens have attracted considerable attention due to their abnormal non-emissive fluorescent behavior in solution but strongly emissive behavior in the aggregate state. By utilizing the inherent hydrophobicity, AIEgens can be used to monitor the crystal formation and dewetting behavior in the self-assembly process. More importantly, some stimuli-responsive AIE-active surfaces have been successfully fabricated. In this perspective review, we outline the advances of surface wettability of AIEgens and its applications.

Introductory lecture: recent research progress on aggregation-induced emission

Since the discovery of the aggregation-induced emission (AIE) phenomenon in 2001, research on AIE molecules has drawn much attention, and this area has been expanding tremendously. This brief review will focus on recent advances in the science and application of AIE molecules, including new mechanistic understanding, new AIE molecules for sensing and imaging, stimuli-responsive AIE molecules and applications of AIE molecules for OLEDs. Moreover, this review will give a perspective on the possible opportunities and challenges that exist in the future for this area.

A novel electron-acceptor moiety as a building block for efficient donor-acceptor based fluorescent organic lighting-emitting diodes

A new electron-withdrawing moiety (BFPz) has been used for the first time as an acceptor in OLEDs and its corresponding core unit (2-Br-BFPz) was synthesized. Combined with an electron-donating moiety triphenylamine, a novel fluorescent material with a D-A structure named TPA-BFPz was synthesized. Encouragingly, the EQE values of non-doped and doped blue OLEDs reach 3.68% and 4.42%, respectively.